Like I understand light is photons. But from whence the photons? How come if I’m in a dark room (by the way, why is the room dark? Why no photons in the room?) and I turn on a flashlight, suddenly there’s photons everywhere?
In: Physics
Light is a disturbance in the electric and magnetic fields that make up the universe.
When you throw a rock into a pond you upset the surface level of the pond. Now there are places where the surface is a little higher than normal and corresponding places where it’s lower than normal.
Light is the same thing. When you see something it’s because your eye is detecting changes in the intensity of the electric and magnetic fields on your retina.
The reason something like a fire creates light is because the intense heat causes electrons to jump up into a high energy state. When they come back down they disturb the electric and magnetic fields around them which then travels in all directions.
Light is indeed photons, but that isn’t the most intuitive explanation for your purposes. Photons are useful when you are talking about quantization of light, or when thinking about light as particles, but it’s better, for your question, to think of light as energy or as waves (yes, light is both a wave and a particle, I know, super weird).
Let’s talk about energy first, because that’s easier to wrap your head around. Light is a type of energy. When you use a device like a torch or a candle, you convert stored energy (chemical energy in the battery or the wax) into light energy. That’s why the dark room has photons of light bouncing around when you turn on the flashlight, and no more light when the battery dies.
Now let’s talk about waves. Think of the entire universe being filled with this substance or field. You can’t touch it, you can’t feel it, it just… exists. Analogous to the atmosphere itself – you don’t have any awareness of it. Until, of course, it is disturbed. If the atmosphere is perturbed and the wind blows against your face, you realise that there is actually some type of medium around you.
Light is very similar. There is a field all around you called the electromagnetic field. When this field is perturbed (think of someone throwing a pebble on to a lake), the ripple that forms and travels out? We have devices that can observe those ripples. Two of them, in fact, and they’re round, squishy, and located in your head (yep, eyeballs). Our eyes have somehow evolved to react to perturbations in this field whenever someone metaphorically tosses a rock into it (which, remember, you can do when you convert energy).
Photons are little bundles of electro-magnetic energy of that operate both as waves & particles. Depending on the amount of energy in the photon, it will vibrate with a different frequency/wavelength and that frequency/wavelength will determine what kinds of matter it can interact with. A small subset of photons have the right frequencies/wavelength to interact with the photo-receptors in our eyes and these photons get referred to as visible light.
So if you are in a dark room, there are lots of photons flying around, but it’s dark because those photons don’t have the right wavelength to light up your eyes. Devices like flashlights are specifically designed to release photons in the visible spectrum so that we can see.
Electrons want to exist in stable configurations known as “energy levels” and when an electrons has too much energy, it release some as photons. This means that when you apply energy into a system, you can make electrons release photons in specific ways. Here are a couple of examples:
1) When a flow of electrons are forced to move through the filament of a tradition light bulb, they transfer some their energy to the filament, which gets turned into a bit of thermal energy (heat) and released as light.
2) When a flow of electrons are forced to move through a LED (Light Emitting Diode), they transfer some of their energy to diode where it gets released as light.
3) Phosphorescence. This one is actually really neat. When the electrons in certain substances absorb photons from the environment, they temporally shift up a partial energy level and stay there for a bit of time before releasing a photon and returning to their original energy level. This is why some substances will light up if you shine a light on them.
Photons are the *force carrier particle* for electromagnetism. What that means is, whenever a particle is affected by the electromagnetic force, something has to “tell” that particle to be affected. Any particle with *charge* like positive protons and negative electrons has to “feel” the force, which comes from photons. Photons represent the charged particles trading around energy.
What that really means is that the photon is giving the particle energy, which usually comes out as momentum. A proton is coming towards another proton, and since they’re both positively charged, they will repel each other. One proton spits out a photon and, as a result, loses momentum flies off in a direction. The other proton absorbs that photon, gains that momentum, and flies off in the other direction.
Photons also energize electrons into higher “orbits” around protons, which is what causes visible light. Proteins in our eyes absorb photons, which energizes an electron in the protein. That forces the protein to change shape, which triggers the nerve to fire and the photon is detected. If electrons absorb photons to go higher, you can probably guess that electrons *emit* photons when they drop into a lower “orbit”. Dropping down means the electron is losing energy, and that energy has to go somewhere. So, the electron spits it out as a photon. Another electron, somewhere, will absorb it.
How lights work depends on the light, but the short version is that through some method they’re pushing electrons into a higher orbit before they drop down and spit that energy out as photons.
Any time an electron absorbs energy, it’ll end up spitting it back out. But, because of entropy, a little less energy gets spit out. Eventually, the photons drop from the energy of the visible spectrum down to the invisible infrared and further down.
Firstly, the room is dark because the photons that were previously in the room have collided with the walls. When this happens, the energy is transformed from light into heat, so the wall gets a little hotter and the room darker. Light moves crazy fast, so it all collides with the walls very promptly, so unless there is a continuous input of new photons, you get darkness
When you turn on a flashlight, the energy stored in the batteries (in chemical form) is transformed into light, basically spewing out a stream of photons
You can use these photons to see because they fly out of the torch, bounce off objects, then fly into your eyes.
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